Electron transfer reactions in biology

Protein matrices are believed to play an important role in the accomplishment of photochemical electron transfer reactions in biological systems. First, the fragments... 

The first cyclic porphyrin dimer (cyclophane porphyrin) linked with two ester groups 15 was synthesized as a model of antenna chlorophyll dimer by condensation of a 2,12-dipropionate porphyrin and a 2,12-bis-(3-hydroxypropyl) porphyrin in high dilution in 1977." In order to clarify the mechanism of electron-transfer reactions in biological systems, a variety of porphyrin dimers have been reported as model systems of parts of the photosynthetic apparatus in the last two decades. The first synthesis of cyclic porphyrin oligomers was reported by Hamilton, Lehn and Sessler in 1986. ... 

Table 18.6 lists formal potentials for common protein electron transfer reactions in biologically related systems. Table 18.7 lists standard reduction potentials for biochemical reduction reactions. 

Experimental Approaches Towards Proton-Coupled Electron Transfer Reactions in Biological Redox Systems... 

Electron-transfer reactions, especially long distance electron-transfer reactions in biological systems, will probably be of primary interest. The influence of the media on the rates of electron-transfer reactions is still poorly understood. It has been established that the electron-transfer reactions can occur at quite high rates when the reactants are separated far beyond the distances of collision. It has been observed that if reactants are linked by an aliphatic chain, the rates can be substantially enhanced. Such studies will be especially pursued in redox reactions involving proteins. 

In conclusion therefore it is felt that electrochemistry does offer a valuable technique to study the electron-transfer reactions of biologically-impor-tant molecules. The mechanisms and products observed electrochemically do appear to be similar in many instances to those of the biological reactions. In cases where the biological products or mechanisms are not known electrochemical studies should prove useful in suggesting potential reaction routes and products. 

D.R. McMillin, Purdue University In addition to the charge effects discussed by Professor Sykes, I would like to add that structural effects may help determine electron transfer reactions between biological partners. A case in point is the reaction between cytochrome C551 and azurin where, in order to explain the observed kinetics, reactive and unreactive forms of azurin have been proposed to exist in solution (JL). The two forms differ with respect to the state of protonation of histidine-35 and, it is supposed, with respect to conformation as well. In fact, the lH nmr spectra shown in the Figure provide direct evidence that the nickel(II) derivative of azurin does exist in two different conformations, which interconvert slowly on the nmr time-scale, depending on the state of protonation of the His35 residue (.2) As pointed out by Silvestrini et al., such effects could play a role in coordinating the flow of electrons and protons to the terminal acceptor in vivo. 

Electron-transfer reactions at ITIES resemble electron-transfer reactions across biological membranes, which adds a special interest. Also, in contrast to homogeneous electron-transfer reactions, they allow a separation of the reaction products. So it is disappointing to report that only very few experimental investigations of electron-transfer reactions at ITIES have been performed. This is mainly due to the fact that it is difficult to find systems where the reactants do not cross the interface after the reaction in addition, side reactions with the supporting electrolyte can be a problem. 

A number of electron-transfer reactions of biological interest have been studied using high-pressure techniques (4, 5). These include the oxidation of L-ascorbic acid by [Fe(CN)6]3- (148), [Fe(CN)5N02]3 - (149), and Fe(phen)2(CN)2] (150). The first two reactions are characterized by volumes of activation of -16 and 10 cm3 mol-1, respectively, which indicate that solvent rearrangement as a result of an increase in electrostriction must account for the volume collapse on going to... 

Proteins containing iron-sulfur clusters are ubiquitous in nature, due primarily to their involvement in biological electron transfer reactions. In addition to functioning as simple reagents for electron transfer, protein-bound iron-sulfur clusters also function in catalysis of numerous redox reactions (e.g., H2 oxidation, N2 reduction) and, in some cases, of reactions that involve the addition or elimination of water to or from specific substrates (e.g., aconitase in the tricarboxylic acid cycle) (1). 

H. Sigel, A. Sigel, Eds., Electron-transfer reactions in Metalloproteins, in Metal Ions in Biological Systems, Vol. 27, Marcel Dekker, New York, 1991. 

Without biological electron transfer reactions (also called reduction/oxidation or redox reactions) life would not exist. Well-organized electron transfer reactions in a series of membrane-bound redox proteins form the basis for energy conservation in photosynthesis and respiration. The basic reaction is simply the transfer of electrons from the donor to the final electron acceptor. Perhaps the best example of these redox reactions, their importance for living organisms, and the nature of the different type of biocatalysts that are involved is the respiration chain present in the membranes of mitochondria. The membrane-bound nature of this electron transport chain, supporting electron transfer from NADH to O2 as... 

Redox-active amino acids are now recognized to play important roles in many biological electron-transfer reactions. In 1988, Bridgette Barry and Gerry Babcock" used EPR spectroscopy to demonstrate the involvement of an isotope-labeled radical in the water-splitting reaction in photosystem II of... 

Isied, S. S. In Metal Ions in Biological Systems Electron Transfer Reactions in Metalloproteins , Sigel, H. Sigel, A., Eds. Dekker New York, 1991 Vol. 27,1-56. 

The experiments referred to in this chapter have also assisted the theoretical analysis of the role of temperature and pressure on biological electron transfer reactions. In the case of cytochrome c, an empirical approach could show that the heme iron is screened more efficiently from surface charges in the oxidized state (37). In general, solute-solvent interactions are directly influenced by temperature and pressure, and these interactions will affect the electrostatic interaction energies, which can be accounted for in terms of changes in the dielectric constant of both the solute and the solvent. Such interactions are of major importance in the understanding of electron transfer processes. 

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